docs/cpp/index.md

# Rust bindings for C++ libraries

When a C++ library enables Crubit, that library can be used directly from Rust.
This page documents roughly what that entails, and additional subpages
(available in the left-hand navigation) document specific aspects of the
generated bindings.

Tip: The code examples below are pulled straight from
<https://github.com/google/crubit/tree/main/examples/cpp/function/>. The other examples
in <https://github.com/google/crubit/tree/main/examples/cpp/> are also useful. If you
prefer just copy-pasting something, start there.

## How to use Crubit {#introduction}

Crubit allows you to call some C++ interfaces from Rust. It supports
[functions](functions.md), [classes and structs](classes_and_structs.md), and
[enums](enums.md). Crubit does **not** support advanced features like templates
or virtual inheritance.

The rest of this document goes over how to create a C++ library that can be
called from Rust, and how to actually call it from Rust. The quick summary is:

1.  Define a `rust_api_from_cpp` target in the same `BUILD` file as your
    `cc_library`.

2.  Add the generated `.hint` target (e.g. `:<name_of_rust_target>.hint`) to the
    `aspect_hints` of your `cc_library`.

3.  Depend on the `rust_api_from_cpp` target in the `deps` of your Rust target.

The bindings can be previewed using the following command:

    ```sh
    $ bazel build --config=crubit-genfiles //path/to:target
    ```

### Write a `cc_library` target {#cc_library}

The first part of creating a library that can be used by Crubit is to write a
`cc_library` target. For example:

```
{{ #include ../../examples/cpp/function/example.h }}
```
<!--  -->


If you write a BUILD target as normal, it will not actually get Crubit bindings,
but we'll start from there:

```
{{ #include ../../examples/cpp/function/BUILD }}
```
<!--  symbol:example_lib_broken -->


### Enable Crubit on a target {#enable}

To enable Crubit on a C++ target, you must define a `rust_api_from_cpp` target
associated with it, and link them using `aspect_hints`.

Behind the scenes, Rust APIs are generated via Bazel aspects which run on the
`cc_library` target. When examining a `cc_library`, Rust API generation looks
for the `aspect_hints` so that it can find the corresponding `rust_api_from_cpp`
target.

Define a `rust_api_from_cpp` target in the same `BUILD` file as your
`cc_library`, and add its `.hint` target to the `aspect_hints` of the
`cc_library`:

```
{{ #include ../../examples/cpp/function/BUILD }}
```
<!--  symbol:\bexample_lib\b -->


```
{{ #include ../../examples/cpp/function/BUILD }}
```
<!--  symbol:\bexample_lib_rust\b -->


The `.hint` target is automatically created by the `rust_api_from_cpp` macro
(named `<name>.hint`) and is used to avoid circular dependencies between the C++
library and the generated Rust API.

Note that having Rust callers does constrain library evolution. Certain changes
cannot be made in C++ without breaking Rust callers, unless care is taken.
crubit.rs/cpp/cookbook#compatibility

### Look at the generated bindings {#examine}

To examine the generated C++ bindings for the target, you can run the following
command:

```sh
$ bazel build --config=crubit-genfiles //examples/cpp/function:example_lib_broken
```

This is the best way to preview the generated bindings for a given C++ target
right now. You might end up using this a lot, so keep it in your shell history.

If you run the above command, you should see some output like the following:

```
Aspect //rs_bindings_from_cc/bazel_support:rust_bindings_from_cc_aspect.bzl%rust_bindings_from_cc_aspect of //examples/cpp/function:example_lib up-to-date:
  bazel-bin/examples/cpp/function/example_lib_rust_api_impl.cc
  bazel-bin/examples/cpp/function/example_lib_rust_api.rs
  bazel-bin/examples/cpp/function/example_lib_namespaces.json
```

These files **are** the generated bindings which are used under the hood when
depending on a C++ target from Rust. They consist of:

1.  The supporting C++ code to glue Rust and C++ together. (The `.cc` file.)
2.  The public Rust interface. (The `.rs` file.)
3.  Supporting internal implementation details. (The `.json` file.)

You don't need to check them in, as they are regenerated automatically whenever
you build a Rust build target which depends on C++.

The `.rs` file is the interesting one for end users. It should contain an
actually useful API for the target:

```
{{ #include ../../examples/cpp/function/example_generated.rs }}
```
<!--  -->



### Use a C++ library from Rust {#use}

To depend on a C++ library from Rust, add the corresponding `rust_api_from_cpp`
target to your Rust target's `deps`:

```
{{ #include ../../examples/cpp/function/BUILD }}
```
<!--  symbol:main -->


At that point, the bindings are directly usable from Rust. The interface is
identical to the `.rs` file previewed earlier, but can be used directly:

```
{{ #include ../../examples/cpp/function/main.rs }}
```
<!--  -->


## Common Errors {#errors}

See crubit.rs/errors

### Unsupported features

Some features are either unsupported, or else only supported with experimental
feature flags
. In
order to get bindings for a C++ interface, that interface must only use the
subset of features currently supported.

The way to work around this kind of problem, in all cases, is to wrap or hide
the problematic interface behind an interface Crubit can handle:

*   Hide unsupported types behind a wrapper. For example, a
    `std::set<T>` is not supported, but a struct which wraps a `set::set<T>` is.
    crubit.rs/errors/unsupported_type describes the process in more detail.
*   Wrap unsupported functions, in general, behind wrappers.
*   **Embed C++ directly in Rust**: Use [**`inline_cpp!`**](inline_cpp.md) to
    allow embedding arbitrary C++ code within a Rust target using the
    `rust_library_with_embedded_cpp` Bazel rule.

[^aspects]: Crubit is an aspect: an automatically generated entity that exists
    on every build target. It is disabled by default, so that Rust
    callers don't accidentally impose on C++ libraries that weren't
    expecting them.

    Aspects allow Crubit to fully understand the dependency graph: the
    bindings for X are in the Crubit aspect of X. This allows Crubit to
    generate bindings which themselves rely on bindings: if a function
    in target `A` returns a struct from target `B`, we know that the
    bindings for `A` will depend on the bindings for `B`. Because Crubit
    is an aspect, it already knows the name of the bindings for `B`:
    it's simply the Crubit aspect on `B`!

    Without aspects, or something like aspects, you would need to write
    down, for every library, the location of its Rust bindings. There is
    no need for that kind of boilerplate when aspects are involved, and
    that is why most things shaped like Crubit use aspects. For example,
    protocol buffers use aspects for their generated implementations in
    multiple languages. (They *also* use named rules, but the rules
    simply re-export the aspect, and the underlying aspect is what is
    used within the rule for referring to transitive dependencies.)
    Thanks to aspects, the `proto_library` doesn't need to re-specify
    "ah, and the Go proto is named `'x'`".

    Be not afraid! Aspects are what make transitive dependencies work
    seamlessly, without boilerplate. So when you see aspect this, or
    aspect that, remember: this is a Good Thing.